Spray coating apparatus

ABSTRACT

An electrostatic, compressed-air spray gun wherein the liquid to be atomized is formed on a conductive support into a film having an extended edge coincident with one edge of a narrow opening through which a high-velocity stream of atomizing air is discharged. The opposite edge of such opening is defined by an electrode between which and the film-support there is maintained an electrical potential difference. The gun may be provided with an insulated spray-surrounding electrode which is maintained at an effective spray-repelling potential by the transmission of ion-borne charges to or from it.

States Patent s] 3,700 168 a Ferrant Oct. 24, 1972 [54] SPRAY COATING APPARATUS 3,219,276 11/1965 Norris... ..239/15X 72 Inventor: Wolfgang Ferram, Bole, Italy 3,317,138 5/1967 Fraser ..239/l5 [73] Assgneei g gf i zfi Ffitm'mfing FOREIGN PATENTS OR APPLICATIONS [22] Filed. Nov 9 1970 1,038,865 8/1966 Great Britain ..239/15 [21] App1.No.: 88,232

R l d U S A r D Primary Examiner-Lloyd L. King 6 ate pp canon am Attorney-Harry M. Downer, MerrIll H. Johnson, [60] Division of S61. No. 826,071, May 12, 1969, David Badgfirand Verne Pat. No. 3,606,972, which is a continuation of Ser. No. 632,583, AprIl 21, 1967, abandoned. ABSTRACT [30] Foreign Application Priority Data An electrostatic, compressed-air spray gun wherein A r 28 1966 German 15 77 737 4 the liquid to be atomized is formed on a conductive April 1966 Germany 77 738's support into a film having an extended edge coin- April 1966 Germany 15 77 739'6 cident with one edge of a narrow opening through p y which a high-velocity stream of atomizing air is discharged. The opposite edge of such opening is defined by an electrode between which and the film- [58] Fie'ld 23q9/3 15 support there is maintained an electrical potential difference. The gun may be provided with an insulated spray-surrounding electrode which is maintained at an [56] References cued effective spray-repelling potential by the transmission UNITED STATES PATENTS of ion-borne charges to or from it.

3,111,266 11/1963 Axleson et a1. ..239/l5 6 Claims, 6 Drawing Figures PATENTEDHBI 24 1912 SHEET 3 OF 3 SPRAY COATING APPARATUS This is a division of application Ser. No. 826,071, filed May 12, 1969, now U.S.Pat. No. 3,606,972, granted Sept. 21, 1971. Application Ser. No. 826,071 is a continuation of application Ser. No. 632,583 filed Apr. 21 1967, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a device for forming a spray of electrically charged liquid particles with the aid of a high-velocity air stream. The device is intended primarily for use in the electrostatic application of sprayed liquid to surfaces in order to form a coating thereon.

Electrostatic spray-coating systems employing compressed air to effect atomization differ in respect to the manner in which the spray particles become charged and also in respect to whether or not they provide an electrostatic field which extends to the surface being coated and which aids in promoting the electrostatic deposition of the charged spray particles on such surface. Some systems, for example that shown in U.S. Pat. No. 1,855,869 to Pugh, contemplate charging the coating material by conduction before it is atomized; while others, for example that shown in U.S. Pat. No. 2,247,963 to Ransburg, et al., first atomize the coating material and then charge the atomized particles by ion bombardment. Some systems, like those of the aforesaid Pugh and Ransburg patents, employ depositing fields extending to the surface being coated; in

others, for example those of U.S. Pat. No. 2,890,388 to Croskey et al. and U.S. Pat. No. 2,894,175 to Lamm, the field provided is largely confined within the spraying device itself and has no significant depositing effect.

The mobility of a charged particle-Le, its response to electrostatic 'forcesvaries inversely as the mass of the particle an directly as the product of the charge on the particle and the strength of the field in which it finds itself. A high charge-to-mass ratio in particles intended to respond to electrostatic forces is therefore a desideratum. ln spray guns using compressed air for atomization, charging of the liquid by conduction prior to its atomization, as by applying a high voltage to a conventional spray gun, has not resulted in a high charge-to-mass ratio in the resulting spray particles, probably due at least in part to the initial formation of relatively large particles which, after separation from the fed stream of charged liquid, are broken up into finer particles. The electrical capacity of a liquid droplet varies directly as its diameter; and when a charged liquid particle breaks up into a plurality of smaller particles the aggregate capacity of the smaller particles is greater than the capacity of the initial large particle, but the aggregate charge on the smaller particles is no greater than that on the initial particle, so that even if the initial particle was fully charged, the smaller particles will be incompletely charged.

Because mere charging of a compressed air spray gun results in incomplete charging of the ultimate spray particles, it is common practice to provide means for charging those particles by bombardment with atmospheric ions produced at a discharge electrode. Whether such an electrode is carried by the gun or located remote from the gun, and whether or not it is arranged to maintain a depositing field, commercial spray-coating systems relying on charging by ion bombardment require the use of relatively high electrical potentials, from a few kilovolts in systems where the charging field is confined within the spraying device up SUMMARY OF THE INVENTION An electrostatic spray gun embodying this invention charges the liquid by conduction before it is atomized, employs a high-velocity air stream for atomizing, and is so constructed as to limit to a small-size the initially formed particles and to charge those particles effectively with the use of relatively low voltages. A preferred form of spray gun embodying the invention has an air passage terminating in a narrow annular discharge opening defined by two concentric electroconductive elements. The inner one of such elements is so constructed that liquid supplied to it near its center is spread into a thin film having a circular leading edge substantially coincident with the inner edge of said annular discharge opening. An electrical potential difference, desirably as close as practicable to that which would cause sparking, is maintained between the two elements defining the annular opening, with the result that a high potential gradient exists at the circular edge of the liquid film. Air discharged at high velocity through the opening atomizes charged particles from the film-edge and forms them into a spray which can be directed toward an article or surface maintained at a particle-attracting potential. Since the spray particles are detached from the liquid film at a point of high gradient, they will bear high charges. The charged spray particles emerging from the gun exert a space charge effect creating an electrostatic field extending to the article or surface being coated and aiding in the electrostatic deposition of particles closer to such article or surface. I

The electrostatic depositing effect can be increased by providing the gun with an insulated (electrically floating), spray-surrounding electrode maintained at spray-repelling potential either by impressing it with ion-transmitted charges of the same sign as those possessed by the spray particles or by causing the emission from it of charges of opposite sign. For example, a stream of ionized air charged similarly to, but independent of, the spray can be directed into contact with theelectrode; the electrode can be provided interiorly with sharp discharge elements providing for the escape from the electrode of charges opposite in sign to the charges on the spray particles; or means may be provided for ionizing the air in the region within the electrode to an extent such that some of the charge on the spray particles passing through that region will be bled off to the electrode. Any of such expedients makes it possible, without the necessity for using voltages higher than that imposed across the annular discharge opening, to maintain the spray-surrounding electrode at a potential such as will promote deposition of the spray on a surface to be coated.

3 DESCRIPTION or THE DRAWINGS FIG. 1 is an illustration, largely diagrammatic in character, illustrating an electrostatic spraying system embodying a hand-held spray gun; a

FIG. 2 is a side elevation on an enlarged scale of the front end of the barrel shown in FIG. 1 with parts broken away and shown in section to illustrate the interior construction;

FIG; 3 is a side elevation in partial section of a spray gun equipped with a spray-surrounding electrode and embodying means for impressing a charge on such electrode by the use of an ionized air stream;

FIG. 4 is a view similar to FIG. 3, but showing a modified electrode and electrode-charging means;

FIG. 5 is a fragmental section, on an enlarged scale, on the line 5-5 of FIG. 4; and

FIG. 6 is a view similar to FIG. 5 illustrating a modified construction.

DESCRIPTION OF THE. PREFERRED EMBODIMENTS The system shown in FIG. 1 comprises a hand-held spray gun designated in its entirety by the reference numeral 10 and including a barrel l1 and a handle 12. Conduits 13 and 14 extending from the lower end of the handle 12 respectively connect the gun to a source 15 of air under pressure and a source 16 of liquid to be sprayed. A voltage lead 17 extends into the handle 12 from one terminal of a voltage source 18, the other temina] of which is grounded as indicated at 19. A trigger 20 mounted on the gun controls the supply of air, liquid, and voltage in any convenient manner. Exterior portions of the gun 10, including the barrel ll, handle 12, and trigger 20 are grounded as through a conductive sheave surrounding, but insulated from, the voltage lead 17. FIG. 1 also shows a grounded article 21 in position to receive spray from the gun.

The barrel of the gun, asshown in FIG. 2, comprises a tubular housing 23 provided at its front endwith-an inwardly projecting radial flange 24 having a central opening 25 through which the spray emerges. Within the casing 23, and conveniently coaxial therewith, are concentric outer and inner tubes 26and 27, the tube 26 being connected to the air line 13 and the tube 27- to the liquid line 14. Mounted on the front end of the tube 26, through the medium of a ring 28 of insulating material, is an annular electrode 29. At their front ends, the tube 27 is enlarged in external diameter and the electrode 29 reduced in internal diameter to leave a narrow annular discharge opening 30 through which the spray emerges for passage through the central opening 25 at the end of the barrel-housing 23. Overlying the enlarged front face of the tube 27 is a cap 32 conveniently supported by an integral stem 33 from within the tube 27. The cap 32 is spaced a slight distance forwardly from the end of the tube 27 to leave at the periphery of the cap a narrow gap through which liquid emerging from the tube 27 can escape in the form of a film having a circular edge substantially coincident with the circular edges of the front face of the tube 27 and of the cap 32.

The inner tube 27, and desirably also the outer tube 26, are electrically connected to the barrel-housing 23 to be grounded as above indicated. The electrode 29 is connected, preferably through a current-limiting re- Sister 34, to the voltage lead 17. Voltage conveyed to the electrode 29 through the lead 17 and resistor 34 creates an electric field extending'across the annular discharge opening 30 to the grounded tube 27 and cap 32. With voltage applied to the electrode 29, and with compressed air andliquid supplied to the tubes 26 and 27 respectively, the liquid is formed into a thin film between the end faced the tube 27 and cap 32, and air emerging at high velocity through the opening 30 atomiz es liquid from the circular edge of such film. Since the annular opening 30 is narrow, having a width equal to only a small fraction of an inch, a strong field can be created between the edge of the film and the electrode 29 with the use. of a relatively low voltage, say

on the order of a few hundred volts. As a result of being formed in a region of high field strength, the liquid particles discharged from the gun are highly charged.

It is to be noted that the size of particles formed by the high velocity air emerging through the opening 30 is limited by the thickness of the liquid film between the cap 32 and the end face of tube 27. Feeding the liquid to the atomizing site in the form of a thin film having an extended edge has advantages over atomizing the liquid from a solid stream such as would emerge from a circular orifice. One such advantage results from the fact that, at the same rate of liquid discharge, the thickness of the film will be substantially less than the diameter of a stream emerging from a circular orifice, with the result of inhibiting the presence of relatively large liquid particles in the discharged spray. Since the size of the particles as initially formed is limited by the narrow gap between the cap 32 and the face of tube 27, and since the particles are separated from the charged liquid film in a region of great field strength, the spray particles will have the high chargeto-mass ratio previously indicated as desirable.

' As compared with the conventional compressed-air spray gun, the atomizing arrangement above described provides a further advantage in that it allows the energy of the air stream to be more efficiently imparted to the liquid in the effecting of atomization. Given the same rate. of air discharge, directing a confined air stream across the edge of a thin film increases the proportion of fine particles in the spray above that attained when the air is directed against the sides of a comparatively thick stream, as in a conventional gun. This feature is of benefit even in a non-electrostatic gun.

Charged spray particles projected from the opening 30 and through the opening 25 in the barrel-housing 23, are attracted to a grounded article being coated. At the same time, mutual repulsion of the similarly charged articles tends to spread the spray, a result which is frequently undesirable. To counteract such spray-spreading tendency in those cases where it is not desired, 1 may provide the wall of the housing 23. at spaced points about its circumference, with a plurality of air-admitting openings 35. The aspirating action of the discharged spray induces a flow of air through the openings 35 and radially inwardly between the flange 24 and the front face of the electrode 29. Such inwardly directed air flow impinges on the spray and tends to prevent it from spreading under the influence of the electrostatic forces acting between the spray particles. To prevent contact with electrode 29 through openings 35, a grounded barrier 22 may be supported within tubular housing 23 between openings 35 and the electrode 29. Y

The dense cloud of spray particles adjacent the gun exerts a space charge effect which creates between it and the grounded article being coated an electrostatic field promoting'the deposition of particles nearer the article. This effect can be supplemented by the action of a spray-surrounding driving electrode mounted on the front end of the gun, as shown in FIG. 3. A sprayrepelling potential applied to the driving electrode will create between it and the grounded article a .field promoting the deposition of charged spray particles on the article and also serving, like the above mentioned inwardly directed air flow, to reduce spreading of the spray as a result of mutual repulsion acting between spray particles. The effectiveness of the driving electrode in promoting deposition and limiting spreading of the spray will depend upon its potential. Most desirably, the driving electrode has a potential above that of the spray passing through it. While the driving electrode might be connected to a voltage source providing such a higher potential, I prefer for reasons apparent above to avoid the use of such a high potential source. That result can be accomplished by insulating the driving electrode from the gun and supplying it with an ion-transmitted charge by means now to be described.

In FIG. 3, the driving electrode 37 is shown as mounted on the gun through the medium of a cup-like member 38 of insulating material. The electrode is hollow-walled, and the space between the walls communicates with a tube 40 of insulating material through which a stream of ionized air is directed. To aid in transmitting electrical charge from such ionized air stream to the electrode, the space between the walls may be filled with some porous material, preferably conductive, such as metal shavings or metal wood 41. Since the electrode is insulated from the gun, and also from ground, the charge which accumulates from the ionized air stream builds up and raises the potential of the electrode until the leakage from it equals the rate at which it receives charge from the ionized air stream.

The air to be ionized may be taken from the air supplied to the gun from the conduit 14 and directed into a valved tube 42 mounted on the barrel-housing 23 exteriorly thereof. The tube 42 communicates with a metal-walled ionizing chamber 43 the walls of which are grounded, conveniently through the tube 42 or other support connected to the barrel-housing 23. Mounted within the ionizing chamber 43 through the medium of a perforated insulator 44 is a pointed ionizing electrode 45 which is connected to an appropriate voltage source. The chamber 43 has a small outlet opening communicating with the tube 40, and the tip of the electrode 45 is mounted as closely as practicable to that outlet opening. Air flowing through the tube 42 and the openings in the insulator 44 becomes charged as it passes the tip of the electrode 45 and flows through the tube 40 into the interior of the driving electrode 37, the walls of which are porous or perforated to the extent necessary to permit the air to escape after surrendering its charge to the electrode. To minimize the leakage of charges from the ionized air stream, it is desirable that the interior of the tube 40 be of small dimension so that the air velocity through it will be high. The work which must be done in moving the gaseous ions from the region of relatively low potential adjacent the discharge electrode 45 to the region of higher potential at the electrode 37 is derived from the kinetic energy of the air stream flowing through the tubes 42 and 40.

The cup-like insulatorsupporting the driving electrode 37 may be provided in its wall with openings 46 for the admission of air serving the same purpose as the air admitted to the barrel-housing 23 through the openings 35 therein; Additional openings 47 for the same purpose may be provided in'the electrode 37.

' Anothermanner of charging an insulated, spray-surrounding driving electrode without the necessity of using a voltage source other than that employed to create the field acrossthe annular opening 30 utilizes the charges on the spray particles passing through the electrode. Assuming that the charges borne by the spray particles are negative in sign, as is preferred, the negative space charge of the dense particle-cloud will repel negative charges in and on the electrode to its outer surface and attract positive charges to its inner surface. By permitting the latter charges to escape from the electrode, the electrode can acquire a net negative charge. One arrangement for so charging the electrode is illustrated inFIG. 4, where the electrode 49 is shown as provided interiorly with inwardly directed, sharply pointed devices 50, the corona discharge from which serves to allow the positive charges to escape to the spray. The net negative charge on the electrode builds up to increase the electrode-potential until, as in the case of the device of FIG. 3, the rate of charging equals the rate of leakage.

Charging the electrode in the manner just indicated involves decreasing the electrical charge on the spray, for the positive charges acquired by the spray from the electrode neutralize some of the negative charge initially borne by the spray; and the net effect is the same as if some of the negative charge on the spray had been bled off to the electrode. However, as explained below, the rateat which the spray loses charge to the electrode can be controlled, it is possible to build up on the electrode a potential high enough so that the strength of the depositing field it creates will more than compensate for the loss of charge suffered by the spray.

The rate of transmission of charge between the spray and the electrode can readily be varied by controlling the ionizing efi'ect of the devices 50, foe example in the manner illustrated in FIG. 5. As there shown, each device 50 is screw-threadedly mounted in an opening 52 in the wall of the electrode. By varying the position of the device 50 radially of the electrode, and/or by varying the extent to which its tip is shielded by the sleeve 51, the extent to which the atmosphere inside the electrode is ionized and rendered conductive can be controlled.

Another electrode-charging means employing the charge on the spray is illustrated in FIG. 6. That means comprises a body 55 of radio-active material capable of emitting positive ions, such as alpha particles, which will ionize the atmosphere traversed by the spray as it passes through the electrode. The body 55 is supported -on the inner side of a plug 56 screw-threadedly mounted in an opening 57 in the wall of the electrode so that the position of the body 55 radially of the electrode can be varied. When positioned near the periphery of the electrode, the body 55 is relatively ineffective to ionize the atmosphere inside the electrode, but its effectiveness will increase as it is moved radially inwardly.

The ionizing devices 50 or 55 should be so adjusted as to obtain a maximum electrostatic depositing effect; and as such depositing effect depends on both the magnitude of the charge on the spray particles and the potential of the driving electrode, the depositing effect can suffer by' transmitting either too much or too little charge from the spray particles to the driving electrode. In considering the manner in which the driving balanced of FIGS. 3 and 4am charged, the electrode and the atmosphere inside it can be regarded as an electrical free body, conductive and insulated from ground. Electrical charge is continuously forced into such free body by the kinetic energy of the atomizing air stream. Part of the charge so supplied leaves on the projected spray and the remainder is transmitted, directly or in effect, through the conductive atmosphere to the electrode serving to raise the potential thereof until the rate at which the electrode acquires charge is balaced by the loss of charge by leakage, as through the insulator supporting the electrode.

I claim as my invention: 1. A method of providing a spray of electrically charged liquid particles comprising providing first and second adjacent elements which cooperate to form an opening and direct a high velocity stream of air, providing a liquid film edge in the opening and carried by one of the elements the liquid film edge being spaced from the other element, establishing a potential difference between the elements creating an electric field across the opening to electrically charge the liquid film edge, and

sweeping the elements with the high velocity stream of air directed by the elements to atoniize the liquid from the film edge on one element into a spray of electrically charged liquid particles carried away from the elements and through the opening. 2. A method of electrostatic-coating an article comprising 1 forming a liquid into film having an edge on a supporting element, 7

creating en electric field having high potential gradient at the edge of the film by positioning a fieldcreating element adjacent the edge of the film, and using thefield creating element and the film supporting element to direct a high velocity stream-of air transversely of the liquid film edge to atomize the liquid into a charged liquid spray and project it away from the elements toward the article to be coated. v 3. The method of claim 1, wherein the potential difference between the element is established by connecting the elements to different terminals of a voltage source.

4. The method of claim 3, wherein element carrying the. liquid film is connected to the ground terminal of the voltage source. I

5. The method of claim 1 including the further step of introducing additional amounts of air to the spray of charged liquid particles to minimize the tendency of UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,700,l68 Dated October 24, 1972 1nvent0r(s) Wolfgang Ferrant It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 40, "front end" should read front and Column 5, line 38 "metal wood" should read metal wool Column 6, line 50, "foe example" should read for example line 53 after "mounted in" insert a conductive sleeve 51 which is in turn screw-threadedly mounted in Column 7, line 14, "balanced" should read electrodes Column 8, line 23, "element" should read elements Signed and sealed this 20th day of March 1973 (SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM (10-69) USCOMM-DC 60376 P69 US. GOVERNMENT PRINTING OFFICE Z 1969 0-365-334, 

1. A method of providing a spray of electrically charged liquid particles comprising providing first and second adjacent elements which cooperate to form an opening and direct a high velocity stream of air, providing a liquid film edge in the opening and carried by one of the elements, the liquid film edge being spaced from the other element, establishing a potential difference between the elements creating an electric field across the opening to electrically charge the liquid film edge, and sweeping the elements with the high velocity stream of air directed by the elements to atomize the liquid from the film edge on one element into a spray of electrically charged liquid particles carried away from the elements and through the opening.
 2. A method of electrostatic coating an article comprising forming a liquid into film having an edge on a supporting element, creating en electric field having high potential gradient at the edge of the film by positioning a field creating element adjacent the edge of the film, and using the field creating element and the film supporting element to direct a high velocity stream of air transversely of the liquid film edge to atomize the liquid into a charged liquid spray and project it away from the elements toward the article to be coated.
 3. The method of claim 1, wherein the potential difference between the element is established by connecting the elements to different terminals of a voltage source.
 4. The method of claim 3, wherein element carrying the liquid film is connected to the ground terminal of the voltage source.
 5. The method of claim 1 including the further step of introducing additional amounts of air to the spray of charged liquid particles to minimize the tendency of the charged liquid particles to spread under the influence of electrostatic forces acting between the charged liquid particles.
 6. The method of claim 5, wherein the additional amount of air is aspirated air. 